Consider Allowing Usage of Custom Types for Nested Attributes

[bendbennett]

Closes: #532

Following changes are included in this PR:

• Introduction of a FrameworkValue interface.
• Moving fwschema/attribute.go to types/attribute.go to avoid import cycles.
• Moving fwschema/nested_attributes.go to types/nested_attributes.go to expose NestedAttributes interface for use in custom nested attribute types.

I am not convinced that attribute.go and nested_attributes.go should be in the types pkg.

[bendbennett]

Closed by #553, #558, #546

[github-actions]

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